1. Field of the Invention
The present invention relates to a printing plate mounting apparatus, and relates in particular to a printing plate mounting apparatus that precisely aligns a printing plate with a plate cylinder and subsequently secures the printing plate to the plate cylinder.
2. Description of the Related Art
Various well known techniques are employed to mount press plates or printing plates (which are materials of the press plates) onto the plate cylinders of printing apparatuses or exposure apparatuses.
But since the sizes of printing lots are steadily becoming smaller, and since there is a continuing growth in the variety of the matter that is being printed, the need is increasing for printing plates (described in this specification as flexible printing plates) that can be produced by depositing layers of photosensitive substances on inexpensive, easily handled supports composed of materials other than metals (e.g., plastic film or paper).
Since a flexible printing plate is less capable of retaining its size and shape than a printing plate for which a metal is used as a support, i.e., it is less rigid, precisely mounting such a flexible printing plate on a plate cylinder is a very complicated process. For this reason, a number of different techniques have been proposed for the mounting of flexible printing plates on plate cylinders.
FIG. 9 is a schematic diagram illustrating one printing plate mounting apparatus 60 for mounting a flexible printing plate on a plate cylinder in a printing apparatus. The printing plate mounting apparatus 60 comprises: a plate cylinder 61, on which a flexible printing plate 70 composed of a flexible material is to be mounted; a rubber cylinder 65, which is positioned adjacent to the plate cylinder 61; and holding mechanism 51, for holding the flexible printing plate 70.
The plate cylinder 61 is shaped as a cylinder from which part has been removed, in the axial direction, to provide a substantially flat face (hereinafter referred to as a flat face 61a) in which a groove is formed. Provided in the groove are a head clamping mechanism 62, for engaging a plate head 70a (that is, a tip end of the printing plate 70, the leading end of the flexible printing plate 70; and a tail clamping mechanism 63, for engaging a plate tail 70b, the trailing end of the flexible printing plate 70. The clamping mechanisms 62 and 63 respectively include: lower jaws 62b and 63b, the top face of each of which lies substantially along the same plane as that of the flat face 61a; and upper jaws 62a and 63a, which are supported rotatably, by the lower jaws 62b and 63b so that the plate head 70a and the plate tail 70b can be clamped between the upper jaws 62a and 63a and the lower jaws 62b and 63b. 
A top portion 61c is formed between the flat face 61a and a curved face 61b of the plate cylinder 61. Although not shown, for some cylinders a top portion is formed between the curved face 61b and the upper face of the lower jaw 62b of the clamping mechanism 62, instead of the flat face 61a. 
Generally, the portion whereat the flat face 61a meets the curved face 61b is rounded to form a curved face having a radius of 10 to 20 mm. In this specification, this portion is also called a top portion.
The rubber cylinder 65 can be brought into contact with or separated from the plate cylinder 61, and when the flexible printing plate 70 is mounted around the plate cylinder 61, it can be held and pressed between the rubber cylinder 65 and the plate cylinder 61.
The holding mechanism 51 is provided for the printing plate mounting apparatus 60 because mounting the less rigid flexible printing plate 70 around the plate cylinder 61 would be an extremely difficult and complicated process were it performed by hand. The design of the holding mechanism 51 provides for the positioning of guide rollers 53 at the distal ends of support members 52 projecting outward from a plate member 50a. 
In the printing plate mounting process, first, an operator loads a flexible printing plate 70 into the holding mechanism 51. Then, as is shown in FIG. 10, while supporting the head 70a of the flexible printing plate 70 the operator guides it between the upper and lower jaws 62a and 62b of the head clamping mechanism 62. The seating of the plate head 70a is adjusted by positioning mechanism, such as resister pins or stoppers (not shown) While thus disposed, the plate head 70a is secured in place by closing the upper jaw 62a in the direction indicated by an arrow A. Thereafter, as the plate cylinder 61 is rotated, the flexible printing plate 70 is wound around the plate cylinder 61.
However, precisely mounting a flexible printing plate 70 on the plate cylinder 61 is still a difficult task. Even when, as is shown in FIG. 10, the head 70a of the flexible printing plate 70 is inserted and positioned between the upper and lower jaws 62a and 62b on the print cylinder 61, as the upper jaw 62 is closed, the head 70a tends to be displaced in the direction (the direction indicated by an arrow B) that causes it to be removed from the head clamping mechanism 62.
When the plate head 70a is displaced, position adjustment mechanism (not shown), provided for the head clamping mechanism 62, can be used to move the head clamping mechanism 62, at the bottom of the groove in the plate cylinder 61, in several directions so as to adjust the position of the plate head 70a. However, at this time, the tail clamping mechanism 63 must be simultaneously moved by the position adjustment mechanism (not shown), and since the operation of the position adjustment mechanism is therefore extremely complex and requires a high degree of skill, this procedure is not practical.
As the result of a thorough study, the present inventors had found the following. When, as is shown in FIG. 10, the portion of the flexible printing plate 70 that is nearer the tail than is the portion that the rotating tip of the upper jaw 62a contacts is bent away from the plate cylinder 61, because of the weight of the flexible printing plate 70, a constant force is exerted that removes the plate head 70a from a position between the upper and lower jaws 62a and 62b. Therefore, when the upper jaw 62 is closed in the direction indicated by the arrow A, the flexible printing plate 70 slides across the rotating tip of the upper jaw 62a, and is displaced in the direction in which the plate head 70a is disengaged from the head clamping mechanism 62 (the direction indicated by the arrow B). That is, since the flexible printing plate 70 is supported by the rotating tip of the upper jaw 62a, which acts as a fulcrum as the upper jaw 62a is closed and the rotating tip moves toward the plate head 70a, the flexible printing plate 70 slides across the rotating tip and is displaced in the direction in which it is removed from the head clamping mechanism 62.
Further, the present inventors have also found that when there is play in the head clamping mechanism 62, the upper jaw 62a moves spasmodically when the upper and lower jaws 62a and 62b are closed, and could result in the displacement of the plate head 70a. 
To resolve the shortcomings, it is one objective of the invention to provide a printing plate mounting apparatus that can precisely and easily mount a printing plate on a plate cylinder.
To achieve this objective, according to the invention, a printing plate mounting apparatus comprises:
a plate cylinder, for which a plate head clamping mechanism having upper and lower jaws is provided to position and secure a plate head of a flexible printing plate; and
a guide member for guiding the flexible printing plate to the plate head clamping mechanism before the plate head is positioned and secured,
wherein the guide member guides the flexible printing plate so that, at least between the guide member and the plate head clamping mechanism, the flexible printing plate is bent toward the plate cylinder, or is substantially parallel to the upper face of the lower jaw of the plate head clamping mechanism.
The forms used for the guide member are not limited, and a guide member can be shaped like a roller or a plate.
A printing plate according to the present invention may include a plate member for which image exposure has not yet performed by an exposure apparatus, and a plate member for which image exposure has been performed, or for which developing has been performed following image exposure, and which can be mounted on the plate cylinder of a printing apparatus. The support member of the flexible printing plate can be composed of a material, other than metal, that is usually employed for offset printing, such as plastic film or paper or a combination of these materials. More specifically, the support member can be a plastic film, such as polyester, polyethylene terephthalate, polyethylene naphthalate or polycarbonate, or a composite sheet formed by the lamination of a plastic sheet, such as polyethylene or polypropylene, and paper. A metal support member may also be employed.
In the thus arranged printing plate mounting apparatus, when positioning and securing a plate head, the guide member steers the flexible printing plate so that, at least between the guide member and the head clamping mechanism, the flexible printing plate is bent toward the plate cylinder, or is substantially parallel to the upper face of the lower jaw,
When the flexible printing plate is bent toward the plate cylinder by the guide member, the head of the printing plate is pressed against the upper face of the lower jaw and is not easily displaced. As a result, positioning of the flexible plate head is not adversely affected by the movement of the rotating tip of the upper jaw. When the flexible printing plate is supported by the guide member so it is substantially parallel to the upper face of the lower jaw, the head of the printing plate is not easily displaced while in contact with the upper face of the lower jaw, which is separated from the upper jaw. Similarly, as a result, the positioning of the flexible plate head is not adversely affected by the movement of the rotating tip of the upper jaw.
Therefore, a flexible printing plate can be secured without its precise positioning being adversely affected.